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MOND[2] is a phenomenological modification of the Newtonian acceleration law. In Newtonian gravity theory, the gravitational acceleration in the spherically symmetric, static field of a point mass at distance from the source can be written as

where is Newton's constant of gravitation. The corresponding force acting on a test mass is

To account for the anomalous rotation curves of spiral galaxies, Milgrom proposed a modification of this force law in the form

However, such conservation laws are automatically satisfied for physical theories that are derived using an action principle. This led Bekenstein[1] to a first, nonrelativistic generalization of MOND. This theory, called AQUAL (for A QUAdratic Lagrangian) is based on the Lagrangian

where is the Newtonian gravitational potential, is the mass density, and is a dimensionless function.

In the case of a spherically symmetric, static gravitational field, this Lagrangian reproduces the MOND acceleration law after the substitutions and are made.

Bekenstein further found that AQUAL can be obtained as the nonrelativistic limit of a relativistic field theory. This theory is written in terms of a Lagrangian that contains, in addition to the Einstein–Hilbert action for the metric field , terms pertaining to a unit vector field and two scalar fields and , of which only is dynamical. The TeVeS action, therefore, can be written as

The terms in this action include the Einstein–Hilbert Lagrangian (using a metric signature and setting the speed of light, ):

where is the Ricci scalar and is the determinant of the metric tensor.

The scalar field Lagrangian is

with , is a constant length, is the dimensionless parameter and an unspecified dimensionless function; while the vector field Lagrangian is

where , while is a dimensionless parameter. and are respectively called the scalar and vector coupling constants of the theory. The consistency between the Gravitoelectromagnetism of the TeVeS theory and that predicted and measured by the general relativity leads to .[4]

In particular, incorporates a Lagrange multiplier term that guarantees that the vector field remains a unit vector field.

The function in TeVeS is unspecified.

TeVeS also introduces a "physical metric" in the form

The action of ordinary matter is defined using the physical metric:

where covariant derivatives with respect to are denoted by .

TeVeS solves problems associated with earlier attempts to generalize MOND, such as superluminal propagation. In his paper, Bekenstein also investigated the consequences of TeVeS in relation to gravitational lensing and cosmology.

In addition to its ability to account for the flat rotation curves of galaxies (which is what MOND was originally designed to address), TeVeS is claimed to be consistent with a range of other phenomena, such as gravitational lensing and cosmological observations. However, Seifert[5] shows that with Bekenstein's proposed parameters, a TeVeS star is highly unstable, on the scale of approximately 106 seconds (two weeks). The ability of the theory to simultaneously account for galactic dynamics and lensing is also challenged.[6] A possible resolution may be in the form of massive (around 2eV) neutrinos.[7]

A study in August 2006 reported an observation of a pair of colliding galaxy clusters, the Bullet Cluster, whose behavior, it was reported, was not compatible with any current modified gravity theories.[8]

A quantity [9] probing General Relativity (GR) on large scales (a hundred billion times the size of the solar system) for the first time has been measured with data from the Sloan Digital Sky Survey to be[10] (~16%) consistent with GR, GR plus Lambda CDM and the extended form of GR known as theory, but ruling out a particular TeVeS model predicting . This estimate should improve to ~1% with the next generation of sky surveys and may put tighter constraints on the parameter space of all modified gravity theories.